Use abstract namespace for domain socket

[sundtek]

Using abstract namespace for the domain socket will fix the stale socket problem when starting rtapi_app.

[sundtek]

sundtek@sundtek-UX32VD:~/devel/mitsubishi-linuxcnc/linuxcnc$ rtapi_app
Note: Using POSIX non-realtime
^C
sundtek@sundtek-UX32VD:~/devel/mitsubishi-linuxcnc/linuxcnc$ rtapi_app
Note: Using POSIX non-realtime
^C
sundtek@sundtek-UX32VD:~/devel/mitsubishi-linuxcnc/linuxcnc$ rtapi_app
Note: Using POSIX non-realtime

before it was like:
(reverse-i-search)`': ^C
sundtek@sundtek-UX32VD:~/devel/mitsubishi-linuxcnc/linuxcnc/src$ rtapi_app
Note: Using POSIX non-realtime
^C
sundtek@sundtek-UX32VD:~/devel/mitsubishi-linuxcnc/linuxcnc/src$ rtapi_app
Waited 3 seconds for master. giving up. <<--- stale fifo
Note: Using POSIX non-realtime
^C
sundtek@sundtek-UX32VD:~/devel/mitsubishi-linuxcnc/linuxcnc/src$ rtapi_app
Waited 3 seconds for master. giving up. <<-- stale fifo
Note: Using POSIX non-realtime
^C
sundtek@sundtek-UX32VD:~/devel/mitsubishi-linuxcnc/linuxcnc/src$ rtapi_app
Waited 3 seconds for master. giving up. << -- stale fifo
Note: Using POSIX non-realtime

a call to rtapi_app in a second window will turn the app immediately to a slave (as intended).
This patch effectively gets rid of the 3 seconds waiting with a stale domain socket.

[andypugh]

Thanks. I don't claim to fully understand it, but it seems to match sample code out there.